Time shifts with faster-than-light photons

GAITHERSBURG, Md. – Experiments with faster-than-light photons are highlighting the weird
world of quantum tunneling. Researchers at the Joint Quantum Institute (JQI) have
boosted single photons to seemingly faster-than-light speeds through a stack of
materials by adding a single, strategically placed layer.

“For the first time, we experimentally show a strange effect
when an increase of the overall structure length leads to a decrease in traversal
time,” said Natalia Borjemscaia, a researcher at JQI, a collaboration of the
National Institute of Standards and Technology and the University of Maryland. “Our
work offers insight into developing nanostructures with tailored optical properties,
particularly with regards to engineering temporal delays and dispersion.”

Natalia Borjemscaia inspects the dielectric stack structure stage. Each sample contains
bare regions used for reference and four regions with different combinations of
starting and ending dielectric stack layers deposited on top of the substrate.
Intuition would have us believe that light achieves its maximum
speed in a vacuum and slows down appreciably when it travels through materials such
as glass or water. We would expect the same to be true for light traveling through
a stack of dielectric materials, but this is not necessarily so. A quantum object
or particle – such as a photon of light – can appear to traverse barriers
of one thickness in less time than it would take to traverse a barrier that is less
thick.

“We investigate how subtle rearrangements of quarter-wave
optical stack layers affect the traversal times of single photons,” Borjemscaia
said. “We show that slight rearrangements of layers can significantly change
the traversal time – by many times what one would expect from the change in
thickness of the structure alone.”

In the experiment, which was published in the February 2010 issue
of Optics Express, a pair of photons is generated using parametric down-conversion,
a process in which one photon from the pump laser is converted into two identical
ones of lower energy with twice the wavelength of the pump.

One photon is sent through one of four sample stacks made with
alternating layers of two materials with different refractive indices: high (H)
and low (L). The other photon is sent through a calibrated delay line.

The JQI team found that traversal times strongly depend on which
layer terminates the structure; in particular, when the group added an H layer to
a structure that initially terminated with two L layers (oneon each end), the structure’s
thickness increased and the traversal time significantly decreased.

This seemingly superluminal, or faster-than-light, speed can be
explained by the wave properties of light. When a photon hits a boundary between
the layers of a material, it creates waves at each surface. The waves interfere
with each other so that very little light makes it out of the other side of the
stack of layers but, provided that the H and L layers are arranged in just the right
way, the photons that do make it to the other side emerge early.

This is the experimental setup used to determine propagation delays
of photons traversing dielectric stack structures. Courtesy of the Joint Quantum
Institute.
Now that Borjemscaia and colleagues have demonstrated the temporal
effects with dielectric stacks, which was one system suggested as a model of the
quantum barrier, they hope to continue their experiments with a different optical
barrier.

“We are actively pursuing measurements of traversal times
through another optical ‘analogue’ of the quantum mechanical potential
barrier – a narrow gap between two blocks of glass,” Borjemscaia said.
“This arrangement is one that better approximates the quantum mechanical barrier
and will fill an experimental void in the direct measurement of tunneling times.”